Scorch-retardant composition

ABSTRACT

The present invention relates to the prevention of scorching before crosslinking of a thermoplastic and/or elastomeric composition with peroxides or azo compounds. This is achieved by using a family of specific nitroxides as additive.

BACKGROUND OF THE INVENTION

[0001] (i) Field of the Invention

[0002] The present invention relates to the prevention of scorchingbefore crosslinking of a thermoplastic and/or elastomeric compositionwith peroxides or azo compounds.

[0003] (ii) Description of Related Art

[0004] Premature crosslinking (scorching) during the preparatory phaseis a major difficulty in the use of peroxides and azo compounds incrosslinking (curing) applications of elastomeric and/or thermoplasticmaterials. The preparatory phase consists in general in blending theconstituents and optionally extruding them at temperatures that areoften high. The operating conditions of this preparatory phase quiteoften lead to decomposition of the peroxide or azo initiator, thusinducing the crosslinking reaction with formation of gel particles inthe bulk of the blend. The presence of these gel particles leads toimperfections (inhomogeneity or surface roughness) of the final product.Excessive scorching reduces the plastic properties of the material, suchthat the said material can no longer be converted, leading to loss ofthe entire batch. In addition, excessive scorching may lead to the totalstoppage of the extrusion operation.

[0005] Several solutions have been proposed to overcome this drawback.Thus, it has been proposed to use an initiator with a halflife of 10hours at high temperature. The drawbacks of this approach are the lowproduction efficiency due to a long curing time and the high energycosts.

[0006] It has also been proposed to incorporate certain additives inorder to reduce the scorch tendency. Thus, the use of organichydroperoxides as scorch inhibitors for polyethylene-based compositionscrosslinked with a peroxide was described in British patent GB1,535,039.The use of vinyl monomers was the subject of patent U.S. Pat. No.3,954,907. The use of nitrites was described in patent U.S. Pat. No.3,202,648. U.S. Pat. No. 3,335,124 describes the use of aromatic amines,phenolic compounds, mercaptothiazole compounds, sulphides, hydroquinonesand dialkyl dithiocarbamate compounds.

[0007] Very recently, the use of 2,2,6,6-tetramethylpiperidyloxy (TEMPO)and 4-hydroxy-2,2,6,6-tetramethylpiperidyloxy (4-hydroxy TEMPO) was thesubject of a Japanese patent application JP 11-49865.

[0008] However, the use of the additives of the art cited above toincrease the scorch-resistance time has a harmful effect on the curingtime and/or on the final crosslinking density. It leads to a reductionin the production efficiency and/or properties of the final product.

SUMMARY OF THE INVENTION

[0009] The present invention eliminates the drawbacks of the art citedabove since it makes it possible to improve the scorch resistance or thecrosslinking density without this having a negative impact on thecrosslinking time. This is achieved by using a family of specificnitroxides as additive.

[0010] A first aspect of the present invention is thus ascorch-retardant composition comprising a nitroxide (N) containing atleast one unsaturation. Nitroxides (N) which may be used, for example,are those represented by the following formulae:

[0011] in which at least one of the substituents R′₁, R′₂, R₁, R₂, R₃,R₄, R₅, R₆, R₇, R₈, R₉ and R₁₀ contains at least one unsaturation whichis reactive towards free-radical addition. The remaining substituents,which may be identical or different, represent a hydrogen atom, ahalogen atom such as fluorine, chlorine, bromine or iodine, a linear,branched or cyclic, saturated hydrocarbon-based group such as an alkylradical, a substituted or unsubstituted aromatic group, a cyano group—CN, an amide group —CONRR_(a), an ester group —COOR, an alkoxy group—OR, a phosphonate group —PO(OR)(OR_(a)), a hydroxyl group —OH or anacid group such as —COOH or —PO(OH)₂ or SO₃H. R and R_(a), which may beidentical or different, may represent a hydrogen atom, a linear,branched or cyclic alkyl group containing a number of carbons rangingfrom 1 to 20, or an optionally substituted phenyl group.

[0012] The substituents containing at least one unsaturation may each inparticular represent a linear, branched or cyclic unsaturatedhydrocarbon-based group or an acrylate, methacrylate, acrylamide ormethacrylamide group. When several unsaturations are present on the samegroup, these may be conjugated, such as, for example, in a butadienylgroup, or unconjugated.

[0013] When the abovementioned formulae also comprise at least onesubstituent containing at least one unsaturation, a hydroxyl groupand/or an acid group, the hydroxyl group and/or the acid group ispreferably represented by R₅, R₆, R₇, R₈, R₉ and R₁₀.

[0014] An aspect of the present invention is also a scorch-retardantcrosslinking composition (C) comprising a nitroxide (N) and afree-radical initiator comprising organic peroxides and azo compounds,and mixtures thereof.

[0015] Another aspect of the invention is a crosslinkable composition(A) comprising a thermoplastic polymer and/or elastomeric polymer whichmay be crosslinked by means of a peroxide or an azo compound, anitroxide (N) and a free-radical initiator comprising organic peroxidesand azo compounds, and mixtures thereof.

[0016] The present invention also provides a process for crosslinking acrosslinkable composition comprising a thermoplastic polymer and/or anelastomeric polymer which may be crosslinked by means of a peroxide oran azo compound, in which the polymer is mixed with a free-radicalinitiator comprising organic peroxides and azo compounds, and mixturesthereof, in the presence of a nitroxide (N).

[0017] The present invention also provides molded or extruded articlessuch as electrical cables or wires obtained from a crosslinkablecomposition (A).

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0018] According to the present invention, a nitroxide (N) of formula(m) is preferably used, in which:

[0019] R₁, R₂, R₃ and R₄, which may be identical or different, representa methyl, ethyl, propyl, isopropyl, butyl, sec-butyl or tert-butylradical,

[0020] R₅, R₆, R₇, R₈ and R₉ each represent a hydrogen atom, and

[0021] R₁₀ represents a linear, branched or cyclic unsaturatedhydrocarbon-based group or an acrylate, acrylamide, methacrylate ormethacrylamide group.

[0022] Mention may be made, for example, of:

[0023] 2,2,6,6-tetraethyl-4-piperidyloxy acrylate

[0024] 2,2,6,6-tetraethyl-4-piperidyloxy methacrylate.

[0025] It is preferred to use, in particular, a nitroxide (N) containingan unsaturation, represented by formula (IV):

[0026] in which X represents a divalent group —OC(O)—, —NR_(b)C(O)— or—P(O)(OR_(b))O— with R_(b) having the same meaning as R and n is aninteger which may range from 2 to 20 and preferably from 2 to 10. Theposition of the unsaturation on the group C_(n)H_(2n-1) is irrelevant,but unsaturations located either in position 1 relative to X or in theterminal position are preferred.

[0027] Mention may be made, for example, of:

[0028] 2,2,6,6-tetramethyl-4-piperidyloxy acrylate

[0029] 2,2,6,6-tetramethyl-4-piperidyloxy methacrylate

[0030] 2,2,6,6-tetramethyl-4-piperidyloxyacrylamide

[0031] 2,2,6,6-tetramethyl-4-piperidyloxymethacrylamide.

[0032] According to the present invention, compounds which may be usedas free-radical initiators are azo compounds and/or organic peroxides,which, upon thermal decomposition, produce free radicals whichfacilitate the curing/crosslinking reaction. Among the free-radicalinitiators used as crosslinking agents, dialkyl peroxides anddiperoxyketals are preferred. A detailed description of these compoundsis found in Encyclopedia of Chemical Technology, 3rd edition, vol. 17,pages 27 to 90 (1982).

[0033] Among the dialkyl peroxides, the preferred initiators are:dicumyl peroxide, di-t-butyl peroxide, t-butylcumyl peroxide,2,5-dimethyl-2,5-di(t-butyl-peroxy)hexane,2,5-dimethyl-2,5-di(t-amylperoxy)hexane,2,5-dimethyl-2,5-di(t-butylperoxy)-3-hexyne,2,5-dimethyl-2,5-di(t-amylperoxy)-3-hexyne,α,α′-di[(t-butylperoxy)isopropyl]benzene, di-t-amyl peroxide,1,3,5-tri[(t-butylperoxy)isopropyl]benzene,1,3-dimethyl-3-(t-butylperoxy)butanol and1,3-dimethyl-3-(t-amylperoxy)butanol, and mixtures thereof.

[0034] Among the diperoxyketals, the preferred initiators are:1,1-di(t-butylperoxy)-3,3,5-trimethylcyclohexane,1,1-di(t-butylperoxy)cyclohexane, n-butyl 4,4-di(t-amylperoxy)valerate,ethyl 3,3-di(t-butylperoxy)butyrate, 2,2-di(t-amylperoxy)propane,3,6,6,9,9-pentamethyl-3-ethoxycarbonylmethyl-1,2,4,5-tetraoxacyclononane,n-butyl 4,4-bis(t-butyl-peroxy)valerate and ethyl3,3-di(t-amylperoxy)butyrate, and mixtures thereof.

[0035] Azo compounds which may be mentioned, for example, are2,2′-azobis(2-acetoxypropane), azobisisobutyronitrile, azodicarbamide,4,4′-azobis(cyanopentanoic acid) and 2,2′-azobismethylbutyronitrile.

[0036] Dicumyl peroxide and α,α′-di[(t-butylperoxy)isopropyl]benzene areparticularly preferred.

[0037] The thermoplastic and/or elastomeric polymers taken intoconsideration in the present invention may be defined as natural orsynthetic polymers which have a thermoplastic and/or elastomeric natureand which may be crosslinked (cured) under the action of a crosslinkingagent. In Rubber World, “Elastomer Crosslinking with Diperoxyketals”,October 1983, pages 26-32, and in Rubber and Plastic News, “OrganicPeroxides for Rubber Crosslinking”, 29 Sep. 1980, pages 46-50, thecrosslinking action and crosslinkable polymers are described.Polyolefins which are suitable for the present invention are describedin Modern Plastics Encyclopedia 89, pages 63-67 and 74-75. By way ofexample, mention may be made of linear low density polyethylene, lowdensity polyethylene, high density polyethylene, chlorinatedpolyethylene, ethylene-propylene-diene terpolymers (EPDM),ethylene-vinyl acetate copolymers, ethylene-propylene copolymers,silicone rubber, chlorosulphonated polyethylene, fluoroelastomers,natural rubber (NR), polyisoprene (IR), polybutadiene (BR),acrylonitrile-butadiene copolymers (NBR) or styrene-butadiene copolymers(SBR).

[0038] Mention may also be made of ethylene-methyl (meth)acrylatecopolymers and ethylene-glycidyl methacrylate copolymers.

[0039] The weight ratio of the free-radical initiator to the nitroxide(N) in the crosslinking composition (C) and in the crosslinkablecomposition (A) is preferably between 1 and 50 and advantageouslybetween 2 and 10.

[0040] In the crosslinkable composition (A), the free-radical initiatorpreferably represents between 0.2% and 5% by weight of the amount ofpolymer and advantageously between 0.5% and 3% by weight.

[0041] In addition to the constituents mentioned above, the compositions(A) and (C) may comprise antioxidants, stabilizers, plasticizers andinert fillers such as silica, clay or calcium carbonate.

[0042] The compositions (A) and (C) may comprise two or more nitroxides(N). They may also comprise two or more free-radical initiators.

[0043] According to the process of the present invention, thecrosslinking temperature may be between 110 and 220° C. and preferablybetween 140 and 200° C.

[0044] Advantageously, the process is implemented in the presence of anamount of initiator and of nitroxide such that the initiator/polymer andnitroxide/polymer weight ratios are close to those of composition (A).

[0045] The conversion of the crosslinkable compositions into molded orextruded articles may be carried out during or after crosslinking.

EXPERIMENTAL SECTION

[0046] In the text hereinbelow, the following abbreviations are used:

[0047] M_(H): the maximum value of the torque obtained from the curverecorded by the rheometer. This value determines the crosslinkingdensity.

[0048] T₉₀: curing time, the time required to reach 90% of the maximumtorque at a given temperature.

[0049] T_(s5): scorch time, the time required at a given temperature forthe torque to increase by 5 Mooney units.

[0050] The crosslinking density (M_(H)) and the crosslinking time (T₉₀)of the blend obtained were measured at 180° C. using a Monsanto ODR 2000E rheometer (Alpha Technologies, oscillation arc: 3, oscillationfrequency: 100 cycles/min).

[0051] The crosslinking time is also determined using the rheometer usedunder the same conditions as above.

[0052] The scorch time was measured at 145° C. using a Mooney MV 2000viscometer (Alpha Technologies).

Example 1 Not in Accordance With the Invention

[0053] 1,000 g of low density polyethylene (Mitene sold by Ashland), 25g of dicumyl peroxide (Luperox® DC) and 3 g of2,2,6,6-tetramethylpiperidyloxy (TEMPO) were mixed together in aturbomixer at 80° C. (nominal temperature) for 15 minutes (stirringspeed=930 rpm). The powder was then converted into a sample in the formof a disc by melting at 110° C. for 3 min. The sample was then placed inthe rheometer or viscometer chamber.

Example 2 Not in Accordance With the Invention

[0054] Example 1 was repeated without the use of2,2,6,6-tetramethylpiperidyloxy. The results are given in Table 1. Thecomparison of Examples 1 and 2 shows that the higher scorch time withTEMPO is accompanied by a large decrease in the crosslinking density anda slight increase in the crosslinking time.

Example 3

[0055] The process is performed as described in Example 1, except thatthe 3 g of 2,2,6,6-tetramethylpiperidyloxy are replaced with 4.4 g ofNT1 (2,2,6,6-tetramethyl-4-piperidyloxy methacrylate) so as to carry outa comparison for an equivalent concentration of nitroxide function.

[0056] The comparison of the results with those obtained in thepreceding examples shows unambiguously that this nitroxide makes itpossible to increase the scorch time while at the same time increasingthe crosslinking density. TABLE 1 Maximum Scorch Crosslinking torque attime at time at 180° C. 145° C. 180° C. (M_(H)) (T_(s5)) (T₉₀) Additive(N.m) (min:s) (min:s) Example 1 TEMPO 1.21 19:00 7:10 Example 2 — 1.95 8:40 6:50 Example 3 NT1 2.42 14:10 6:20 Example 4 — 5.94  2:50 5:21Example 5 OH-TEMPO 5.14 16:19 5:39 Example 6 NT1 6.14 15:47 4:12

Example 4 Not in Accordance With the Invention

[0057] 318 g of compound EPDM DIN 7863 (containing 100 g ofethylene-propylene-diene terpolymer and 218 g of fillers) wereconditioned in a Banbury-type mixer with a volume of 350 cm³ at 50° C.for 5 minutes at a speed of 50 revolutions/min. 8 g of Luperox F40ED(40% di(tert-butylperoxyisopropyl)benzene and 60% inert fillers) wereadded and mixed with the compound for 5 minutes at 50° C. at a speed of50 revolutions/min.

Example 5 Not in Accordance With the Invention

[0058] Example 4 is repeated but with the addition not only of theLuperox F40ED but also of 0.677 g of4-hydroxy-2,2,6,6-tetramethylpiperidyloxy (OH-TEMPO).

Example 6

[0059] Example 5 is repeated but adding, instead of the OH-TEMPO, 0.931g of NT1.

What is claimed is:
 1. A scorch-retardant crosslinking compositioncomprising a nitroxide (N) containing at least one unsaturation and afree-radical initiator comprising an organic peroxides, an azo compound,or mixtures thereof.
 2. The composition of claim 1, wherein thenitroxide (N) is represented by formulae (I) to (III):

in which at least one of the substituents R′₁, R′₂, R₁, R₂, R₃, R₄, R₅,R₆, R₇, R₈, R₉ and R₁₀ contains at least one unsaturation which isreactive towards free-radical addition; the remaining substituents,which may be identical or different, represent a hydrogen atom, ahalogen atom, a linear, branched or cyclic, saturated hydrocarbon-basedgroup, a substituted or unsubstituted aromatic group, a cyano group —CN,an amide group —CONRR_(a), an ester group —COOR, an alkoxy group —OR, aphosphonate group —PO(OR)(OR_(a)), a hydroxyl group —OH or an acidgroup; R and R_(a), which may be identical or different, may represent ahydrogen atom, a linear, branched or cyclic alkyl group containing anumber of carbons ranging from 1 to 20, or an optionally substitutedphenyl group.
 3. The composition of claim 2, wherein said halogen atomis fluorine, chlorine, bromine or iodine
 4. The composition of claim 2,wherein said saturated hydrocarbon-based group is an alkyl radical. 5.The composition of claim 2, wherein said acid group is —COOH or —PO(OH)₂or SO₃H.
 6. The composition of claim 2, wherein the substituentscontaining at least one unsaturation each represent a linear, branchedor cyclic unsaturated hydrocarbon-based group or an acrylate,methacrylate, acrylamide or methacrylamide group.
 7. The composition ofclaim 1, wherein the nitroxide (N) is represented by formula (IV):

in which X represents a divalent group —OC(O)—, —NR_(b)C(O)— or—P(O)(OR_(b))O— with R_(b) having the same meaning as R and n is aninteger ranging from 2 to
 20. 8. The composition of claim 7, wherein nranges from 2 to
 10. 9. The composition of claim 7, wherein thenitroxide (N) is: 2,2,6,6-tetramethyl-4-piperidyloxy acrylate,2,2,6,6-tetramethyl-4-piperidyloxy methacrylate,2,2,6,6-tetramethyl-4-piperidyloxyacrylamide, or2,2,6,6-tetramethyl-4-piperidyloxymethacrylamide.
 10. The composition ofclaim 1, wherein the free-radical initiator is dicumyl peroxide orα,α′-di[(t-butylperoxy)isopropyl]benzene.
 11. The composition of claim1, further comprising a thermoplastic polymer and/or an elastomericpolymer.
 12. The composition of claim 11, wherein the polymer is a lowdensity polyethylene, a high density polyethylene, a chlorinatedpolyethylene, an ethylene-propylene-butadiene terpolymer, anethylene-vinyl acetate copolymer, an ethylene-propylene copolymer, asilicone rubber, a chlorosulphonated polyethylene, a fluoroelastomer, anethylene-methyl (meth)acrylate copolymer or an ethylene-glycidylmethacrylate copolymer.
 13. The composition of claim 1, wherein theweight ratio of the free-radical initiator to the nitroxide (N) isbetween 1 and
 50. 14. The composition of claim 13, wherein the weightratio is between 2 and
 10. 15. The composition of claim 11, wherein thefree-radical initiator represents between 0.2% and 5% by weight of theamount of polymer.
 16. The composition of claim 15, wherein the freeradical initiator represents between 0.5% and 3% by weight of the amountof polymer.
 17. A process for crosslinking a crosslinkable compositioncomprising a thermoplastic polymer and/or an elastomeric polymer whichmay be crosslinked with a peroxide or an azo compound, comprising thestep of mixing said polymer with a free-radical initiator comprising anorganic peroxide, an azo compound, or mixtures thereof, in the presenceof a nitroxide (N) containing at least one unsaturation and afree-radical initiator comprising an organic peroxides, an azo compound,or mixtures thereof.
 18. The process of claim 17, comprising convertingthe crosslinkable composition into molded or extruded articles during orafter crosslinking.
 19. Molded or extruded articles obtained accordingto the process of claim
 18. 20. The composition of claim 2, wherein Rand R_(a) may represent a cyclic alkyl group containing a number ofcarbons ranging from 1 to 20, or a substituted phenyl group.